1. Field of the Invention
The present invention relates to a signal level crossing detector circuit for use in particular but not exclusively in mains voltage powered consumer products.
2. Description of the Related Art
Mains voltage powered consumer products, such as multimedia home networking nodes, are required for reasons of safety to have electrical isolation between mains voltage circuitry and low voltage circuitry. Despite the electrical isolation there is often a need to convey signals across the electrical isolation barrier between the mains voltage circuitry and the low voltage circuitry. The determination of a location in time, such as a zero crossing point, on the mains voltage signal from the low voltage side is an example of such a need involving the conveyance of signals from the high voltage side to the low voltage side. The determination of a location on the mains voltage signal finds application, for example, in providing for synchronization with a mains voltage cycle. Synchronisation with the mains voltage cycle may be used to provide for synchronized communication between and amongst low voltage circuits of multiple networked products, such as multimedia home networking nodes.
A more specific example is the provision of an ISP (Inter-System Protocol) that provides for cooperation between and amongst devices that are operating according to different communications protocols. The ISP involves opening a window once every predetermined number of zero crossings of the mains voltage signal and the sending of a recognised symbol followed by control data that provides for the cooperation. The determination of a location on the mains voltage signal may also find application in monitoring the phase variation of a mains supply to determine whether or not the mains supply is liable to fail. For example, there may be an increased likelihood of failure in supply if there is more than a 2% variation in the phase of a mains supply from a twenty-four hour mean. Precautionary measures may then be taken, such as the engagement of an uninterruptable power supply. According to a more simple application, the determination of a location on the mains voltage signal may be used to determine whether the mains voltage signal is of 50 Hz or 60 Hz. In yet a further application, the determination of a location on the mains voltage signal may be used to adapt the communications channel to optimise channel capacity. More specifically, mains voltage signal location determination may be used as the basis for dividing the mains voltage cycle into parts, with various noise indicative characteristics, such as power level and expected signal to noise ratio, in the different parts being monitored and compared in different communications links. The channel is then adapted in dependence on the comparison of the noise indicative characteristics.
A known electrical isolator circuit 10 for conveying signals from a mains voltage circuit to a low voltage circuit whilst maintaining isolation between the mains and low voltage circuits for subsequent zero-crossing detection is shown in FIG. 1. The electrical isolator circuit 10 comprises an opto-isolator 12 having an infrared light emitting diode (LED) 14 and a photo-transistor 16. A resistor, Rin, 18 in series with the LED limits the current flowing through the LED. A load resistor, Rout, 20 is present in series with the photo-transistor 16 between the photo-transistor and the positive power line. A capacitor, Cload, 22 represents a parasitic capacitance of the electrical circuit connected to the output 24 of the photo-transistor. In use, a high voltage AC signal is applied across the inputs 26 to the electrical isolator circuit to thereby cause operation of the LED 14. Light emitted by the LED is received by the photo-transistor and causes a current to flow in the photo-transistor with the current developed across the load resistor to thereby provide a corresponding voltage at the output 24. A representative high voltage AC signal 28 is shown in FIG. 2 along with a corresponding output voltage 30 from a zero-crossing detector that takes as its input the voltage signal from the output 24. The high voltage AC signal 28 and the corresponding output voltage 30 are not to scale in FIG. 2. More specifically and for example, the high voltage AC signal 28 might be 230 Vrms and the output voltage 30 might be 3.3 volts. In other embodiments, the AC signal 28 might be 110 Vrms or 120 Vrms.
A disadvantage of a zero crossing detector including the electrical isolator circuit of FIG. 1 is its high power dissipation on the low voltage side and, in particular, on the high voltage side. The temporal accuracy of the output 24 depends on the speed at which the isolator circuit 10 is capable of switching with the switching speed being determined by the RC time constant of Rout and Cload. The maximum value of Rout can be determined for a given load and a desired accuracy. The maximum value of Rout and the required voltage swing in turn determine the minimum required photo-transistor current, Ic. The LED forward current, If, is then determined on the basis of the current transfer ratio (CTR) of the opto-isolator having regards to the photo-transistor current, Ic. For high voltage signals, most of the voltage is dropped across Rin. A power dissipation of 0.5 Watts can be expected for a typical opto-isolator and typical values for Rout and Rin. If the forward current, If, is reduced to a significant extent to reduce the power dissipation there is not only a corresponding reduction in the photo-transistor current but also a reduction in the current transfer ratio of the opto-isolator. The combination of these effects results in a much more significant reduction in the output swing, which necessitate an increase in Rout, which in turn decreases the switching speed of the isolator circuit by a corresponding amount. Hence, a zero crossing detector including the electrical isolator circuit of FIG. 1 presents an unacceptable compromise between power dissipation and switching speed.